1. Field of the Invention
This invention relates generally to brakes and more specifically to fluid pressure operators for road vehicles.
2. Description of the Prior Art
Generally, hydraulic brake systems for road vehicles include a master cylinder connected in fluid communication with wheel cylinders located at the vehicle wheels. Fluid pressure is generated in the system at the master cylinder which pressure acts on the wheel cylinders to actuate the brakes. The pressure is generated at the master cylinder due to the application of an externally produced force, such as by force exerted on a brake pedal. The master cylinder generally comprises a reservoir and a main cylinder integratedly formed in a housing. Ports are provided to fluidly connect the reservoir and the main cylinder. Hydraulic fluid generally fills the brake system including the master cylinder reservoir, the main cylinder and the wheel cylinders. A piston is reciprocable in the main cylinder. When the brake pedal is depressed, the resulting force displaces the piston which creates pressure in the system acting through the fluid to actuate the brakes via the wheel cylinders. Pressure in the system is enhanced due to a resilient sealing cup adjacent one end of the piston.
One of the aforementioned ports which fluidly connects the reservoir and the main cylinder is generally referred to as a compensating or by-pass port which allows for relatively small increases and decreases in the volume of the fluid due to temperature changes by bleeding off excess fluid pressure from the main cylinder back into the reservoir. When the piston is displaced the annular periphery of the cup sealingly moves along the annular cylinder wall of the main cylinder. During this movement the cup passes over the compensating port. The system, being subjected to increasing pressure during piston displacement, and the cup being resilient, results in the increased pressure extruding a portion of the cup into the compensating port. Commonly, the edges of the port then cut off the extruded portion of the cup from the annular periphery. This is commonly known as cup cutting and causes leakage in the master cylinder which can cause eventual brake failure due to incapacity of the master cylinder to generate the required pressure in the brake system. The fluid used in such hydraulic systems is generally non-compressible and is consequently very responsive to the pressure applied.
In drum brake systems, the cup cutting problem is probably minimal. This is due to the fact that in such systems there is generally greater displacement of the piston since there is substantial clearance between the brake shoes and the brake drum and this clearance is directly proportional to the hydraulic displacement of the piston and fluid. The greatest portion of the piston displacement occurs at low pressure during the inintial pressure increase in the system so that the cup is usually past the port before the system is subjected to significant pressure.
In disc brake systems, however, the cup cutting problem is significantly increased since there is generally less clearance involved and thus less total displacement of the piston in the cylinder. In this case a significant pressure buildup occurs rapidly so that significant pressure exists when the cup moves through the cylinder across the port.
In the past, this problem has been reduced by incorporating a displacement cylinder in the system external to the master cylinder. The purpose of such a displacement cylinder is to absorb to take up the initial rapid increase in pressure during displacement of the piston to reduce initial pressure increase in the main cylinder until the piston and cup are displaced across the compensating port. Such displacement cylinders generally include a piston biased by a substantially low force spring so that upon initial application of the brakes, fluid from the master cylinder is absorbed by the displacement cylinder so that initial pressure increase is diverted from the main cylinder to the displacement cylinder while the piston experiences its normal displacement in the main cylinder. This in effect limits the pressure rise rate with respect to the stroke of the piston. Such external displacement cylinders cause valuable weight and space limitations in master cylinder construction and installation and can cause rather costly and complex innovations in master cylinder design and construction. In the interest of avoiding the above-mentioned limitations and complexities, it would be advantageous to provide means within the master cylinder, rather than external thereto, for the purpose of assuming rapid initial pressure increases associated with actuated master cylinder pistons.